The present invention relates to a procedure apparatus for monitoring the current data in a thyristor-controlled d.c. elevator drive, the operating system of the invention comprising two thyristor bridges converting alternating current into direct current for the drive motor, and a current measuring unit.
The elevator may be in zero-current state in one of two cases, namely, when there is a fault in the operating system or when the load imposed causes a normal, transient zero-current state. It is difficult to distinguish between these two states. As a result, a dangerous situation is caused to those using the elevator and, in particular, also to the personnel servicing and repairing the elevator. Therefore, in some countries the elevator regulations in fact demand monitoring of the d.c. circuit. It has however been difficult, as a rule, owing to the said reason to arrange for the monitoring of current because it is not possible to observe whether the motor is or is not supplied with current. After the developing of semiconductors to replace earlier types of equipment, new needs have arisen in the current monitoring systems. By virtue of their advantageousness, thyristors have in ever more numerous instances replaced the dynamotor current converters. Thyristor drives work with a considerably better efficiency, and therefore the operating costs are better. The manufacturing costs likewise are reduced from those of rotary converters. But certain new detriments have also appeared in these drive systems. It is a frequent event in a normal elevator operation that the usual situation exists wherein the drive motor moves the elevator car and therefore requires current is in fact inverted in that by effect of the loading of the elevator, of the direction in which it moves and of its speed the elevator car "pulls" the drive motor. The drive motor will then operate as a generator and it will supply current to the mains network instead of drawing current therefrom. In such situations the current is reversed and, due to the nature of the drive systems, the system remains for a moment in a complete zero-current state at the time of change. The duration of the zero-current state varies and it has not been possible by known techniques to determine its length exactly. Measurement of current could not be applied in any satisfactory way because emergency stops would occur altogether too frequently and without cause at current reversals in the system.
In a procedure of prior art, a fuse was connected in the circuit between the thyristor bridges and the drive motor. This fuse is monitored with the aid of an optoisolator so that when the fuse goes the optoisolator transmits the information that the supply of current to the motor is interrupted. Emergency stop is actuated on the strength of this information. This system has the drawback that one is only able to monitor current supply trouble due to failure of the fuse. If the current is interrupted for any other reason, the system is insensitive to the event as regards the monitoring operations.